There has been considerable information published on the production of microbial protein. The term `microbial protein` has developed two meanings. One meaning connotates the whole cell, in which the protein is contained within the confines of the cell wall and therefore is relatively nonfunctional. The other meaning connotates a protein isolated as a separate entity from the microbe. In either case, for human nutrition, the nucleic acid content of the protein product should be reduced to below 9 percent if a substantial amount of yeast protein is used in the human diet. The Recommended Daily Allowance of The Food and Nutrition Board, National Research Council for protein is 65 grams per day for a 70 kilogram adult male and The Protein Advisory Group of the United Nations System recommends that the amount of nucleic acid ingested per day from microbial protein should be less than two grams. Therefore, the nucleic acid content of the protein should be less than 3 percent if these criteria are to be met when yeast protein is the only source of dietary protein. The nucleic acid content should be less than 6 percent if the yeast protein constitutes 50 percent of the dietary protein.
The nucleic acid content of yeast cells such as Candida utilis and Saccharomyces cerevisiae is about 12 to 15 grams of nucleic acid per 100 grams of crude protein. Crude protein is calculated in this application as the Nitrogen (N) content multiplied by 6.25. The protein isolated from these cells also contains 12 to 15 grams nucleic acid per 100 grams of crude protein. Thus, the nucleic acid content must be reduced substantially, up to four to five fold, before the protein can be considered as acceptable as the sole source of protein for human nutrition. The nucleic acid of yeast is mainly ribonucleic acid or RNA, and in this application these terms will be used interchangeably.
The reduction of the nucleic acid content can be accomplished by the hydrolysis of the nucleic acid within the cell to fragments of such size that the fragments can be diffused from the cell away from the protein. It is known that the enzyme, nuclease, is present in certain yeast cells and that nuclease hydrolyzes or breaks up nucleic acid molecules to smaller fragments. It also is known in the art that the hydrolysis of nucleic acids within the cell can be accomplished by a multi-step heating process to activate the self-contained or endogenous nuclease to produce cells containing two to three grams of nucleic acid per 100 grams of protein. Nucleic acid also can be hydrolyzed by exposing the cell to an external nuclease. It is further known in the art that alkali can be used to extract nucleic acid from yeast cells.
In any of these procedures, two fractions are obtained. One fraction is the cell containing a reduced content of nucleic acid. The other fraction is the surrounding medium containing nucleic acid fragments and other diffusable material. One disadvantage of these processes is that the protein remains within the cell in a non-functional form for food use. Another disadvantage is that the processes by which the cell wall is made permeable to the nucleic acid fragments also severely decrease the ability of the cell to be ruptured to allow the protein to be harvested. A further disadvantage is the difficulty in controlling the endogenous protease which hydrolyzes the protein, thereby complicating protein recovery.
When yeast cells from which the RNA has not been separated are ruptured by any method, a cellular debris fraction and a soluble cytoplasmic constituent fraction are obtained. These fractions can be separated by centrifugation or filtration. Among the soluble cytoplasmic constituents are the nucleic acid and the protein, either individually or in conjugation. In any situation, recovery of the protein by isoelectric precipitation results in a protein product with an undesirable content of nucleic acid.
We have discovered a process by which thermal energy can be used to prepare a protein product having a reduced nucleic acid content in good yield from yeast. The protein product also has desirable functional characteristics. The heat treatment process steps may be applied at any of several places in the process used to produce yeast protein. The heat treatment of this invention may be applied effectively to ruptured yeast cells; to the soluble cytoplasmic fraction from ruptured yeast cells; or to recovered isolated protein product. In each of these instances, the RNA is effectively separated from the protein.
Another advantage of this process is that the protein can be recovered without additional treatment. A third advantage is that the cell walls of yeast can be recovered as a separate valuable product. Still another advantage is that the non-protein solubles cytoplasmic constituents can be recovered and processed to a valuable product. Because of the severity and extent of the heat treatment, a still further advantage of this invention is that the protein and the soluble constituents are recovered free of microorganisms, i.e., are sterile.